Passive Q-switched or mode-locked pulse light source based on continuous-wave excitation has a configuration that includes a laser medium (Nd:YVO4, etc.) and a saturable absorber. In particular for the case where a short pulse is required or a higher recurrence frequency is desired, both are often used in contact with each other in order to shorten the resonator length (see Patent Document 1, for example).
Referring now to a small-sized pulsed laser which is constituted of a laser medium and a saturable absorber, the resonator length can be shortened and stabilized by thinning the device thickness through use of a semiconductor saturable absorber such as SESAM (semiconductor saturable absorber mirror) or SBR (saturable Bragg reflector), and this is preferable for the case where a higher recurrence frequency is desired or a shorter pulse is preferred. This sort of laser resonator is usually configured using a pair of flat mirrors having no curvature, and formed as a stable resonator making use of thermal lens effect within the laser medium.
FIG. 8 is a graph which exemplifies an excitation light power dependence of lens focal length based on the thermal lens effect, where a relation between both is expressed while plotting input power “Pin” (unit: W) on the abscissa, and focal length “f” (unit: mm) on the ordinate.
As illustrated in the figure, decreasing trend of f with increase in Pin can be found.
A resonator using the thermal lens effect based on temperature dependence of the refractive index can be formed by making use of temperature rise at around the excitation center, which is ascribable to conversion of non-oscillating energy to phonon or re-absorption of light as by-products of absorption of the excitation light at an irradiated region of the excitation light.
FIG. 9 is a graph which exemplifies a profile of temperature rise in the radial direction of the laser medium (Nd:YVO4) caused by the excitation light, where a relation between the both is expressed while plotting the radius “r” (unit: mm) assuming the excitation center as a reference position on the abscissa, and plotting relative temperature change “□T” (unit: K) assuming temperature at r=0 as a reference temperature on the ordinate (temperature decreases in the direction from the excitation center portion towards the peripheral portion).
It is found that the excitation light of approximately 1 W condensed in the laser medium results in a temperature rise of approximately 200 K at the excitation center portion (r=0).
Such temperature rise may elevate temperature of the saturable absorber disposed in close contact with the laser medium and may undesirably vary the characteristics thereof, and this consequently makes it difficult to increase the output. In short, influence of heat or a high-temperature region generated in the laser medium to the saturable absorber may become a problem.
To avoid this problem, there is known a configuration in which an air layer or an intermediate layer is disposed between the laser medium and saturable absorber (see Patent Documents 2, 3 and 4, for example), and this makes it possible to reduce thermal influence (degree of heat conduction) of temperature rise of the laser medium exerted on the saturable absorber.
FIG. 10 schematically shows an example of this type of configuration “a”, in which the laser medium and saturable absorber are disposed in a separated manner.
An excitation light emitted from an excitation light source “b” advances through an optical system “c” to reach a substrate “d” and irradiate a laser medium “e”.
A saturable absorber “f” opposed to the laser medium “e” is formed on a substrate “g”, and a gap “h” is formed between the laser medium “e” and saturable absorber “f” so that air can exist therebetween. The opposing planes of the laser medium “e” and saturable absorber “f” are kept in parallel, and this allows variation in the resonator length (light path length) through adjustment of the length of the gap “h”.
Patent Document 1: Japanese Patent Application Publication No. 2001-185794 (p. 4-7, FIGS. 1 and 7);
Patent Document 2: Japanese Patent Application Publication No. 2000-101175 (p. 7-8, FIG. 4);
Patent Document 3: Japanese Patent Application Publication No. 2001-358394 (p. 3-5, FIGS. 1, 3 to 5); and
Patent Document 4: Japanese Patent Application Publication No. 11-261136 (p. 6-8, FIGS. 1 and 2).
The above-described configuration, however, suffers from a problem of degradation of stability due to vibration and time-dependent changes.
In the configuration having the laser medium and saturable absorber individually fixed on the independent substrates, a change in the resonator length of as much as approximately one-fourth of the oscillation wavelength due to vibration or expansion of an adhesive under temperature change can vary the effective gain due to changes in the oscillation wavelength and relative position of gain spectrum, and this considerably varies the characteristics such as output and pulse recurrence frequency. This configuration also tends to be readily affected by mechanical vibration and causes the jitter to increase, and is associated with a problem that only a small change in the resonator length due to time-dependent changes may result in a large variation in the operating point such as output and pulse recurrence frequency (degradation in the stability).
The configuration having an intermediate matter such as a spacer or shim disposed between the laser medium and saturable absorber is successful in avoiding direct heat transfer from the laser medium towards the saturable absorber, but the heat transfer via the intermediate matter towards the saturable absorber raises another problem of temperature rise of the laser medium or saturable absorber which generally have only a small heat transfer coefficient. Necessity of using a thin spacer also raises a problem (infiltration, etc.) caused by surface tension of the adhesive during adhesion, and raises difficulty in the fabrication.
It is therefore a subject of the present invention to improve, with respect to a laser light generating device, the stability against vibration and time dependent changes, and to reduce influences of temperature changes on the resonator.